1. Field of the Invention
This invention relates to a radio communications system making multi-carrier radio transmission from base stations to a plurality of mobile stations to execute a frequency band allocation control and an application control to modulation and coding.
2. Description of the Related Art
In a conventional radio communications system, a mobile terminal measures a radio transmission path quality, determines a receivable transmission format (combination of modulation and coding rates) on the basis of the measurement value, and transmits this information to a base station as CQI (Channel Quality Indication) (refer to, for example, 3GPP TS 25.214 V5.11.0 (2005-06) 6A HS-DSCH-related procedures).
The base station selects a transmission format based on the CQI, and transmits the transmission information over an individual information channel. The kind of the individual information channel is transmitted immediately before transmission of the transmission information as the control information. The mobile terminal receives information about the transmission format of the individual information channel, over a control channel, and receives the transmission information in accordance with the transmission format.
In such a radio communications system, if an error occurs in the reception of the control channel at the mobile terminal, the reception cannot be conducted since a correct transmission format of the individual information channel is unknown. For this reason, an error tolerance of the control channel needs to be sufficiently greater than an error tolerance of the individual information channel.
In addition, an OFDM (Orthogonal Frequency Division Multiplexing) system is one of multi-carrier systems making communications simultaneously employing a number of subcarriers (refer to, for example, 3GPP TSG RAN1#40bis R1-050249 Downlink Multiple Access Scheme for Evolved UTRA Multiplexing of Common Channel and Shared Data Channel). A specific frequency can be allocated to each mobile terminal, by each subcarrier. This system is similar to the conventional FDM (Frequency Division Multiplexing) system from the viewpoint of allocating a specific frequency to each mobile terminal. In the multi-carrier system such as the OFDM system, however, each mobile terminal receives all the multi-carrier subcarriers simultaneously, executes receiving processing of multi-carrier signals and takes out subcarriers to be received. The OFDM system is different from the conventional FDM system in this point.
A group of subcarriers is called a chunk and the subcarriers are allocated by chunk to the terminal, in 3GPP TSG RAN1#40bis R1-050249 Downlink Multiple Access Scheme for Evolved UTRA Multiplexing of Common Channel and Shared Data Channel. A single user can use a plurality of chunks. In the following descriptions, too, a group of subcarriers is called a chunk.
At present, however, a concrete scheme has not been clarified in relation to a configuration of the control channel to transmit the transmission format information and transmission band information of the individual information, in the radio communications system employing the frequency band allocation control and the application control for modulation and coding on the basis of the transmission path quality measured by the mobile terminal.
The settings for the reception of the individual information channel are obtained over the control channel. If the mobile terminal is not able to normally receive the control channel, the mobile terminal cannot receive the individual information channel. In other words, even if the reception quality of the individual information channel is sufficient, the reception of the individual information channel is insufficient when the reception quality of the control channel is insufficient.